The invention relates to a liquid crystal display device comprising a layer of liquid crystal medium, sets of electrodes disposed on opposing sides of the layer to which energising voltages are applied, the liquid crystal medium being switchable between at least a first state in which visible light incident from a front side of the layer is back-scattered and a second state in which said incident light is transmitted through the liquid crystal medium to the rear side, and a light absorber disposed at the rear side of the liquid crystal medium layer for absorbing the visible incident light transmitted therethrough. The invention relates also to apparatus including the display device.
An example of such a display device, and in particular a so-called paper white display device, is described in PCT WO 94/04958. The liquid crystal medium in this display device consists of a polymer dispersed liquid crystal (PDLC) film comprising a liquid crystal mixture forming microdroplets in an optically isotropic, transparent, polymer matrix. In the PDLC film, one refractive index of the liquid crystal mixture is chosen to be around the same as that of the cured polymeric matrix containing the liquid crystal microdroplets and differs from that of the cured pure polymer. In the unenergised, off, state in which no voltage is applied to the electrodes on opposing sides of this film, the liquid crystal molecules in the microdroplets adopt a distorted alignment and visible light incident thereon is back-scattered at the phase boundary between the polymeric and liquid crystal phases. In effect, therefore, incident light is reflected back in the general direction of the incident light so that to a viewer on that side the film takes on a white appearance under ambient lighting. When an energising, on, voltage is applied to the electrodes, the liquid crystal molecules in the microdroplets are aligned parallel to the field and perpendicular to the E vector of the light and the film becomes an optically isotropic medium for light incident generally perpendicularly of the film. The liquid crystal medium thus becomes transparent and incident light is transmitted through the film and is then absorbed by a black, light absorbing, back plane at the other, rear, side of the film. To the viewer the display device appears dark. In this way, and with the display device comprising an array, for example a row and column matrix, of individually energisable display elements, the display device can provide a display output consisting of dark, and particularly black, characters or graphical information on a white background rather like a conventional black-on-white printed page. In addition to the display appearance, such a display device has a number of advantages, including the fact that, unlike conventional transmissive and reflective T.N. type LC display devices, no polarisers are required and, unlike transmissive devices, a dedicated back-light is unnecessary which makes the display device particularly suitable for use in portable equipment, such as notebook computers, requiring battery power and in which the power consumption of the normally required back-light is significant.
A display device of this kind can be produced conveniently as a large area display device. It has been proposed that large area PDLC display devices, sometimes also referred to as microencapsulated NCAP display devices, can be used as outdoor display boards. For smaller display size applications such as computer displays, the display device can comprise a matrix array of individually operable display elements. The array can be of an active matrix addressed type in which each display element is driven via an associated switch device or of the simple passive matrix type.
Another black on white display device using a polymer dispersed type LC medium together with a black back plane is described in the paper entitled "Polymer-Stabilised Cholesteric Texture Materials for Black-on-White Displays" by J. L. West et al published in SID 94 Digest at pages 608 to 610. In this display device, the display elements are similarly switchable between highly reflective and transparent states. The PSCT liquid crystal material offers bistability and the display elements are switched between the two, stable, states by the application of voltage pulses.